Researchers worldwide have galvanized to combat SARS. The challenge for public health is to determine the nature and potential of any viral threat and to frame a battle plan.

"We are in an evolutionary stage of this epidemic," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. "It could plateau, go up and down, disappear or explode. This virus is highly virulent, potentially lethal and highly transmissibleit has the potential to cause a really bad epidemic."

Currently the death rate from SARS hovers around five and a half percentwhich makes it about 50 times more deadly than the average annual flu. But as diagnostic tests for SARS improve, says Fauci, medics may find that many more people have been infected by the virus, with no serious consequences, and the death rate will drop.

The World Health Organization has identified the nature of SARS. The U.S. Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, and the British Columbia Cancer Agency in Vancouver, Canada, have cracked the genetic code of the virus.

"It's encouraging that we have the rapid identification of a completely novel virus, the proof of causality, the sequencing of its genetic code, the generation of preliminary diagnostic tests and initial drug screening," Fauci said.

A Deadly Combination

SARS is a so-called hybrid virusthe kind that always triggers a red alert. Traditionally a virus affects a single species. But sometimes two viruses combine their genetic material and form a new virus that "jumps" to another species altogether.

Hybrids are dangerous because the body has never encountered them before, and the immune system is unprepared.

"From studying the sequence we see that the SARS virus is derived from a mouse coronavirus and an avian coronavirus," said Michael Lai, a pioneer in coronavirus genetics at the University of Southern California in Los Angeles.